This invention relates to an emergency access method in a centralized monitoring system utilizing power supply wiring which enables the system to respond quickly to an emergency situation such as fire, leakage of gas and burglary.
As is well known in the art, data transmission over power supply wiring is usually carried out in sychronism with the utility power frequency since there are various kinds of noise traveling over the power supply wiring and in synchronism with the utility power frequency. Such transmission is slow because each of the data bits is assigned to each of the power supply frequency cycles. Coaxial cable or wireless transmission, each of which involve a rather complex procedure to achieve improvement of reliability, have not been available.
Two simpler methods are the time-sharing fixed assignment method or the polling method as shown in FIGS. 1 and 2. The time-sharing fixed assignment method, as shown in FIG. 1, permits each of the terminals to transmit data 1-5 in its own time slot after the recognition of a specific code H (referred to as "header") from a central system. The polling method, as shown in FIG. 2, permits only one terminal, selected by address signals (polling signals) P.sub.1, P.sub.2, P.sub.3 and so forth, to transmit data D.sub.1, D.sub.2, D.sub.3 and so forth. Both of the conventional methods however suffer from the problem that it takes a substantial amount of time for the central system to collect all of the data when there are more than several tens of the terminals installed. The most serious problem is that the system is unable to respond quickly to any emergency request included in the data being transmitted.